1. Field of the Invention
The present invention relates to a device for evaluation of the lubricating characteristics of a drilling mud.
During drilling for oil, mud under pressure is sent from the surface to the bottom of the well through the inside of the drill string and the mud then comes back up to the surface through the annular space defined between the string and the well wall.
The drilling mud is intended to lubricate and to cool the drilling tool which is driven in rotation at the bottom of the well. It is also used to bring the spoil back up to the surface. Finally, a function of the mud is to keep the well under pressure in order to prevent untimely arrivals of various components liable to be present under pressure in the well.
2. Description of Related Art
Drilling muds may be oil- or water-based. However, there is a tendency to replace oil muds, which had the advantage of having a high lubricating power, with water-based muds, for environmental reasons. It is appropriate, therefore, to improve the lubricating nature of a water-containing mud by adding additives to it. In order to have the ability to select the most suitable additives for improving the characteristics of a water mud, it is necessary to know its behaviour in the high pressure and high temperature conditions which prevail at the bottom of a wellbore.
The rock which surrounds a wellbore is generally porous, and as a result of this the liquid constituents of the mud tend to enter the rock, leaving solid particles to form a deposit on the well wall. The presence of this deposit of mud may be detrimental for the progress of the drilling, for two reasons. During the drilling it happens that the drill string, and more particularly the drill collars which are of larger diameter, come to rub against the well wall, where the presence of the mud deposit tends to brake the rotation of the string. This results in abrasion and overheating of the drill string and in higher energy consumption to keep the string in rotation. Moreover, when the string ceases to rotate for a prolonged period, it may happen that a phenomenon of jamming of a part of the string will occur in this mud deposit. In order to free the string, a certain traction force and a torque must be applied to it. An apparatus for analysing the lubricating characteristics of a drilling mud must make it possible to take these various phenomena into account, or even to reproduce them.
An apparatus intended to measure the force necessary to release a drill pipe previously jammed in a mud deposit created on an internal surface of the apparatus is known. The drill pipe is applied to the deposit with a force and for a period which are predetermined and the force needed to release the drill pipe is then measured. However, since the measuring sensors are arranged on the drill pipe outside the vessel of the apparatus, the friction produced on the drill pipe by the seal joints has a considerable effect on the measurements which are taken.